The present invention relates to an information recording medium, an information recording and reproducing method, and an information recording and reproducing apparatus, and in particular to an information recording medium, an information recording and reproducing method, and an information recording and reproducing apparatus suitable for optical disks and optical disk drive apparatuses using the sample servo method.
Conventionally, in optical disk apparatuses, the tracking method has been frequently used. In the tracking method, a groove is provided on a disk and control is exercised so that an optical beam may be positioned on this groove. In this case, a control error signal is continuously obtained by detecting light diffracted by the groove from reflected light of the optical beam, and this method is called the continuous servo method. This continuous servo method has a problem that a difference in shape and reflectance between grooves affects the control error signal and worsens the tracking precision. At the time of recording, an optical beam having a larger quantity of light than that at the time of reading is incident and hence the quantity of reflected light is increased in proportion to the quantity of incident light. This necessitates such a contrivance as to obtain a proper control error signal.
In the sample servo method, however, a sample servo format is used instead of the groove as means for obtaining the control error signal, a servo section and a data section are alternately disposed on tracks of a disk, and control is so exercised that an optical beam may be passed over the servo section. As for the control error signal, one set of tracking pits (two tracking pits) disposed so as to be offset around the track position in directions which are opposite to each other are recorded beforehand in the servo section. By detecting the difference between quantities of light reflected by the disk when the optical beam passes through both pits, the control error signal is obtained. In the sample servo system, therefore, there is no influence due to the groove because there is no groove. Further, the control error signal is derived only from the servo section, and only the operation for obtaining servo information is conducted in this area. Therefore, the influence due to the increase of the quantity of laser light at the time of recording is avoided. Some problems of the continuous servo method are thereby solved.
On the other hand, information recording and reproducing apparatuses such as optical disks and magnetic disks are demanded to have larger capacities with an increase in quantity of information to be dealt with. For this purpose, the recording density on disks must be increased. In the so-called CAV (Constant Angular Velocity) method of conducting a recording/reproducing operation while rotating a disk at a constant angular velocity, the space between pits (pit period) is limited at the innermost circumference where the linear velocity becomes the slowest. Therefore, the out the circumference is located, the larger is the allowance between pits. In a circumference having a diameter which is twice that of the innermost circumference, for example, the recording density is decreased to 1/2. Thus, in the CAV method, the recording capability of the disk is not used sufficiently, resulting in waste.
In contrast, the CLV (Constant Linear Velocity) method of keeping the linear velocity constant produces the same recording density from inner circumferences to outer circumferences and hence provides the maximum recording capacity. In the CLV method, however, the angular rotational velocity of the disk must be changed according to the track position in order to keep the linear velocity constant and hence the access time becomes longer than that of the CAV method. This hinders its use as a computer memory, of which high-speed access performance is required. As one of methods aiming at eliminating drawbacks of both methods, therefore, an apparatus in which a disk is divided into a plurality of areas in the radial direction and different angular velocities are set for respective areas so as to increase the recording density, is described in JP-A-61-172223. Further, an apparatus in which a disk is divided into a plurality of areas in the same way and the recording density is increased by changing the pit period in each area while keeping the angular velocity constant is described in JP-A-61-175968. However, neither of them makes mention of the tracking method.
Further, as an example of an apparatus for dividing a disk into a plurality of areas in the radial direction and conducting a recording/reproducing operation in the same way, JP-A-1-128276 can be mentioned. This relates to an optical disk apparatus using the sample servo method. Over the entire face of the disk, the sample period of servo information is not changed but is fixed. Only the frequency of the data section in each of a plurality of divided areas is changed so as to increase the recording density of each area. Reference may further be made to JP-A-1-204272, JP-A-2-162578, JP-A-2-260285 and JP-A-3-187072 published on Aug. 15, 1991.